Marinus Sebastus was a scion of the dynasty of the Sergi (Dukes of Naples) and the Amalfitan family of the Capuano. He was a sebastos

Sebastos

Sebastos was an honorific used by the ancient Greeks to render the Roman imperial title of Augustus. From the late 11th century on, during the Komnenian period, it and variants derived from it formed the basis of a new system of court titles for the Byzantine Empire. The female form of the title...

who was elected Duke of the Republic of Amalfi in 1096 in opposition to Norman

Normans

The Normans were the people who gave their name to Normandy, a region in northern France. They were descended from Norse Viking conquerors of the territory and the native population of Frankish and Gallo-Roman stock...

 suzerainty.

Bohemond of Taranto and Roger I of Sicily

Roger I of Sicily

Roger I , called Bosso and the Great Count, was the Norman Count of Sicily from 1071 to 1101. He was the last great leader of the Norman conquest of southern Italy.-Conquest of Calabria and Sicily:...

 attacked Amalfi

Amalfi

Amalfi is a town and comune in the province of Salerno, in the region of Campania, Italy, on the Gulf of Salerno, c. 35 km southeast of Naples. It lies at the mouth of a deep ravine, at the foot of Monte Cerreto , surrounded by dramatic cliffs and coastal scenery...

 but were repulsed. It was at this siege that Bohemond met travelling warriors on the First Crusade

First Crusade

The First Crusade was a military expedition by Western Christianity to regain the Holy Lands taken in the Muslim conquest of the Levant, ultimately resulting in the recapture of Jerusalem...

 and left to join them with an army. After his victory, Marinus strengthened the defences of the city and added 20,000 Saracen

Saracen

Saracen was a term used by the ancient Romans to refer to a people who lived in desert areas in and around the Roman province of Arabia, and who were distinguished from Arabs. In Europe during the Middle Ages the term was expanded to include Arabs, and then all who professed the religion of Islam...

troops to the navy. He also created the ordo curialium, a court of justice, and recognised the autonomy and democracy of the citizenry.

Marinus was finally deposed by the Normans in alliance with certain Amalfitan noblemen sometime between 1100 and 1110.